1. Field of the Invention
The present invention relates to a catalyst, for rendering the harmful components of exhaust gases harmless and eliminating them, and a manufacturing process thereof. More specifically, the present invention relates to a monolithic catalyst coated with a film of active alumina and oxides of rare earth elements for the purpose of rendering the harmful components of exhaust gases from internal combustion engines, i.e., CO, HC and NO.sub.x harmless, and a manufacturing process thereof.
2. Description of the Prior Art
As is well-known, a monolithic carrier, particularly the so-called honeycomb carrier having a number of fluid paths each partitioned by thin walls, is characterized in that is possesses a low resistance to the passage of exhaust gas on account of its high porosity; it possesses a good warm-up property on account of less heat dissipation from its casing than in the case of a granular catalyst; and it suffers little physical water for structural reasons. These features makes this carrier suitable for a catalyst to purify the exhaust gas from an internal combustion engine which is exposed to heavy vibrations and exhaust pulsations. However, such a monolithic carrier is usually fabricated of an inorganic refractory material such as cordierite, mullite, .alpha.-alumina, silicon nitride or a heat-resistant metal to make it resistant to the high heat of exhaust gas; and such a refractory carrier which carries a catalyst on its surface usually cannot exhibit those properties which are practically required.
The reason for this is that the refractory material to constitute the monolithic carrier is treated at a relatively high heat to develop the required strength; and, in consequence, the specific surface area diminishes, which results in a decreased diffusibility of the catalyst metal into the carrier.
Meanwhile, a commonly used granular carrier to carry a catalyst for exhaust gas purification is mainly composed of active alumina and by virtue of its large specific surface area and appropriate distribution of its pores it is free from the drawbacks of the monolithic carrier. However, it has defects not known in the monolithic carrier such as wear through mechanical vibration, because it is granular, and poor warm-up property, because it is charged into a casing with high thermal capacity.
As disclosed in U.S. Pat. No. 3,565,830, it is known to coat the monolithic carrier with a film of a catalytically effective active alumina, thereby reforming it into a catalyst carrier with higher catalytic activity and/or higher durability; and the relevant technique of coating is also disclosed in the related literature.
Additionally, Japanese Patent Application Laid Open No. 14600/1973 teaches a method of preventing alumina transfer due to high heat through addition of an oxide of rare earth metal to active alumina.
As known from British Patent Specification No. 950235, alumina-copper oxide-oxide of rare earth element is effective as an application of an oxide of rare earth element as a catalyst for purification of exhaust gas from the internal combustion engine.
These conventional methods, however, cannot fully meet the practical requirements for catalyst function.
For instance, even when following the Japanese Patent Application Laid Open No. 14600/1973, if an oxide of a rare earth element is simply added for stability to alumina, the catalytic component will not be successfully stabilized. When a cerium oxide alone is added, the performance of the catalyst in purifying HC's will rather deteriorate.
If a mixture of oxides of rare earth elements is employed, the coating strength will drop or the ion adsorption capacity will drop when the catalytic component is carried with the use of an aqueous solution of salts of platinum group metals. Thus, a satisfactory performance of the catalyst cannot be expected.
The stability of alumina can well be attained even when less than 2 weight % of an oxide of rare earth element is added to alumina; but the effect of a cerium oxide or a lanthanum oxide to promote the catalytic reaction or improve the diffusibility of a carried precious metal salt will not be sufficient.
As for a catalyst of a base metal oxide such as a transition element oxide or a rear earth element oxide as disclosed in British Patent Specification No. 950235, it lacks durabilty and is unfit for practical use.
A need therefore continues to exist for a durable catalyst which can effectively purify the exhaust gases of an internal combustion engine, i.e., substantially eliminate CO, HC and NO.sub.x.